Choose your fantasy:
Genre of overarching story: Mythic Fantasy. Gods of old are returning to Earth to take over as a part of an ancient organization's plot to rule the world, with only 5 heroes as the hope of defeating them both.
Genre of scene: Celtic Fantasy. Three of the heroes get trapped in a city in Faerie and have to rely on the treacherous fae to ever escape to save the world.
Enlarging your world:
"The city took what humans consider to be facts of reality and turned them on their heads. It was a city where a building of stone was coaxed into the shape of a tree, and where a tree was shaped to be a home that looked like a rock formation with ever-growing outcroppings. Clear glass, glittering gold, gleaming silver, refracting crystal, added on wherever the creators of the buildings decided they’d look pretty in places and ways that no human craftsman could put with tools. Some houses obviously started out as white trees but were formed into unique shapes, still growing and rooted even as they provided shelter, often large enough that the tree should fall over, accentuated with metals at random locations to give the wood some shine and glass windows seeming to be naturally part of the structure. Other buildings were made out of stone that seemed to naturally come out of the ground below like a natural rock formation. They could see that even gravity was disregarded, with paths to the buildings around them growing to match entrances that opened to the sides or even the ceilings of buildings as often as they allowed people in at the floor. Many of the paths scorned convenience in favor of whimsy, going in curves, circles, and even upside down before they reached their destinations. Despite the variety, one thing was consistent: the fog that was the city’s namesake enveloped everything. It wasn't thick enough to hide anything, instead leaving it all with a somewhat pleasant blur. Occasionally colors-deep blues, verdant greens, blood reds, and occasional glints of many others-would ripple throughout, with no apparent source. The colors refracted through the crystal, went through the glass.
What's Up People:
the race living in my city are the beings from Celtic myth known as the fae, because for my book series especially this specific race I am solely relying on the lore there's...not much in the aspects the article suggests. Traditionally they are wearing ornate Shakespearian garb, are taller and thinner than humans, and have pointed ears.
The fae are masters of their environment and wielders of illusion magic (glamour) and conjuration, they don't have jobs but live as aristocrats who find amusement with human lives which they consider toys, they don't really need to eat and there's never any description of farms or whatever because food is magically conjured at their feasts (feasts which if a human eats from they will keep eating and hungering for the food of the fae forever, character in my book has this happen it's sad), their clothing is glamoured to look like it does...however, the "environment" of Ceo is essentially a trap they use on humans, as it reduces their inhibitions with its narcotic fog and thus makes us easier to play with.
For culture I'll copy/paste the 7 aspects in the article and answer that way
Social Organization (family units and social classes): traditionally the fae seem to mimic the human royal courts (Hence being called the Seelie or Unseelie courts), so you have the Royalty on top, the courtiers close to the throne, and commoners. Family determines class, the fae don't have the same emotional bonds that we do when it comes to family.
Customs and Traditions: Decadence and debauchery are the hallmarks of fae society. Their nature is too fluid for traditions or customs of the sort. In order to deal with the boredom of immortality they crave novelty and fun...and the best source for both of them is playing with humans, wreaking havoc on our lives which seems evil to us, but they consider us like toys to be played with not as actual human beings. In Ceo they lure humans into the city as its fog slowly wears away at the human's ability to resist or control themselves, playing with them until the fog wears away everything in their minds and renders them useless.
Religion: The fae are semi-divine beings themselves, nature spirits. They recognize the existence of the Tuatha De Danaan, the Celtic gods, not based on faith but because they actually interact sometimes, though they usually keep to themselves.
Language: Technically Gaelic however they have the ability to both understand and be heard in every language at once.
Arts and Literature: In Ceo the fae can shape their homes and the materials they use, essentially making the whole entire city a gallery of their art each one of them bringing their strange vision to life.
Governing Systems: Monarchy.
Economic Systems: Don't really have one, no need for one.
So You Want To Play With Magic:
Who can use it: In the larger world the magic varies with both each religion having its own magic system and also people being able to draw upon nature (I'm not really inventing anything instead drawing upon the rich preexisting cultures and traditions). The nature based one is unique because with the Return of the Gods the magic of nature has also grown back to the levels it had in myth, meaning that neopagans and other modern witches have experienced a huge surge in the powers they have called upon. In the group the leader is Áilleacht, who is descended from one of the ancient Celtic druids that escaped the Romans slaughtering them, however I'm basing my depiction of her on actual historical druids which weren't at all like the nature magic controlling archetype we associate with the term, and there's Atsa who is a Navajo (Diné as they actually call themselves) shaman guided by a spirit that has decided to remain unnamed to him (I know who it is but he's...wily [description and a hint]). The fae have their own innate magic which I already explained.
What does it require: For humans it depends, honestly when it comes to these aspects I'm still partway through researching and gathering info to bring to life. For fae nothing.
Where does it come from: Áilleacht, nature and ancient druidic traditions; Atsa, the spirit that chose him to be a guide to the group; fae from their supernatural nature.
How do people respond to it: If humans revealed that they had such connections it depends on who they're with. The people who reject the Gods and want the world returned back the way it was before their Return with hostility, showing that they have foreign power in places serving particularly controlling Gods will have negative results, places where the power is from and accepted they would be respected, and I imagine the common person who is just going along with the flow would shrug and accept that this is a thing now. The fae are viewed with fear and respect, you don't want to be picked up by them, it's why the Irish have invented a ton of different euphemisms other than fae because simply saying that word attracts their notice. If you're actually in the know about things carrying iron because it's the only thing that can hurt them and avoiding faerie rings is a good reaction to knowing how their magic works.
What does it do?/What can't it do?: Humans: still researching, fae, make illusions of any sort, eating their food or dancing with them traps you into doing those things until death, conjurations, and in Ceo shape the city however they want.
What Makes Us Who We Are:
Alex is a seemingly stereotypical muscle man from Greece with a dark past. In Greece he was a sailor with his partner as part of a fishing crew. However, out in the seas they ran across the sirens, whose enchanting song lured everyone to their deaths except Alex who remembered how Odysseus and his crew were told to plug their ears with beeswax. Lacking beeswax he put his earbuds in and blasted music at max volume so he couldn't hear the sirens...only to watch as all of his friends and the man he loved jumped off the edge of the boat to follow the song and drowned. After seeing how the presence of the Gods and their monsters caused such death, when Atsa came telling him of an opportunity to seal them away again he jumped for the chance so nobody else would suffer like he did. Because homophobia is rather rampant in Greece he doesn't reveal he isn't straight, so being tall, tan, and muscular from hard work sailing leads to women fawning over him which makes him deeply uncomfortable. He's trapped in Ceo because after Jon, a character who had most of his mind erased by the force opposing the Gods that Alex thinks they should ditch to be able to better save the world, said fae once which brings their attention Alex decided in his rather rash nature to say fuck it and repeat the word multiple times leading to the fae picking them up because they were actually willing to forgive Jon and not take them but Alex basically made them want to teach him a lesson. Because his first exposure to mythology coming back to the world was death of the guy he loved he's highly distrustful of the magic and everything else that came back with the Return.